We have found that lesion of a hormone-sensitive forebrain nucleus (the magnocellular nucleus of the anterior neostriatum: MAN) in male zebra finches disrupts song learning in juveniles, but has no effect on production of already-learned song patterns by adult birds. We also know that the time course of lesions in MAN parallels the development of song as a motor pattern, and that the total volume of this nucleus decreases dramatically during this time. Experiments to be conducted will investigate: (1) The complete afferent and efferent neuroanatomical connections of MAN during vocal development. Knowing the transsynaptic pathway from the periphery to this forebrain nucleus will help determine what kind of information is being processed there. Knowing if the afferent and/or efferent projections change during development will help explain why this nucleus increases in volume. (2) Steroid autoradiography experiments will determine whether there are developmental changes in hormone accumulation by cells in MAN and other (monosynaptically connected) song-control nuclei. Because cells in MAN accumulate androgens in adult birds, we think it is likely that the function subserved by this nucleus depends on hormones. (3) Fluorescent dyes will be used to mark cells in MAN with a long-term label. This information will enable us to determine whether the decreased volume in this nucleus is attributable to neuronal death. (4) A developmental Golgi analysis of cells in various song-control nuclei will be performed in order to assess potential changes in dendritic morphology that relate to vocal learning. (5) Electrophysiological experiments will be conducted to obtain chronic multi-unit recordings from MAN and other song-control nuclei during various stages of vocal development. The results of these experiments will greatly enhance our understanding of neural and hormonal mechanisms of vocal learning.